The long-term goal of this study is to address the problems of elbow joint dysfunction and reconstructive surgery. Biomechanically, it is important to characterize 1) the mechanical environment and requirements imposed on the elbow joint in daily and recreational activities, and 2) the pathologic motion loss, weakness, and instability, thereby defining the functional impairment to be corrected or addressed by reconstructive surgery. A detailed analysis of the reconstructed elbow joint under these mechanical environments will allow definition of the objective benefits of the surgery and provide a basis for selecting a given procedure. The specific aims of this study are: (1) to analyze the muscle and joint forces across the elbow joint resulting from selected activities of daily living and (2) to compare the constraint and stability of five types of prostheses for elbow joint replacement. To achieve these two specific goals, both the analytic and experimental methods developed in past years will be used. The kinematics and external load application on the segments distal to the elbow joint will be collected for 10 selected activities by combined use of the electromagnetic tracking system, electrogoniometers, and load cells. The intersegmental forces and moments about the elbow joint due to inertial effects as well as external loading will be calculated using the three-dimensional DYNAMIC model developed. Finally, the specific muscle and joint resultant forces and moments will be calculated using the MUSCLE model. This study will be accomplished by using 40 normal subjects, 20 of each sex ranging in age from 50-60 years. Joint constraint and stability performance will be used to compare the five selected designs of prostheses for elbow joint reconstruction and replacement, each of which is currently available and represents a unique design feature and concept. Experimentally, the joint stability or laxity in cadaveric specimens with prosthetic replacements will be quantitated by using the joint kinematic analysis. Movement of the elbow joint will be achieved with simulated muscle loading as well as additional valgus-varus stress. Changes in the pattern and magnitude of the axes of rotation will be used to assess the joint constraint. In addition, the mechanical advantage of each muscle around the replaced joint will be calculated based on the tendon excursion and joint rotation. Sensitivity analysis will also be performed to analyze the effects of surgical placement of these prostheses and soft tissue reconstruction.